This invention relates to forming a hermetic resistance weld between zinc-coated steel members and to improving corrosion protection of external surfaces where the zinc coating is disrupted by the welding process. More particularly, this invention relates to resistance welding zinc-coated steel members wherein at least one member includes a copper plate intermediate the steel substrate and the zinc coating.
Resistance welding is employed for forming a lap joint between thin steel members. An electrical current passing between the members resistively heats the faying surfaces and fuses the steel. The current is passed between opposed electrodes in contact with exterior surfaces of the joint. Resistance between the electrode and the member generates heat also at the contact surface.
When steel members having metallic zinc coatings for corrosion protection are resistance welded, it has been found that zinc vaporizes from the faying surfaces prior to fusion of the steel substrates. Zinc may also vaporize from the electrode contact surface, thereby exposing the steel at an outer surface that is susceptible to corrosion. The vaporized zinc forms a black smut that deposits on nearby surfaces, including the electrodes. Smut accumulation, and the increased electrode cleaning necessitated thereby, may be one reason that relatively short electrode life has been reported for resistance welding zinc-coated steel.
In some applications, a resistance weld is desired not only to join the members together, but also to form a hermetic seal. However, there is a tendency for microscopic fissures to form in the fused steel, which fissures may extend through the weld and permit air leakage. Formation of a hermetic weld between zinc-coated steel is complicated by the process by which zinc is vaporized and expelled from between the faying surfaces.
Therefore, it is an object of this invention to provide an improved method for resistance welding a zinc-coated steel member to produce a hermetic joint and to protect the steel against corrosion, particularly at external surfaces, such as at electrode contact surfaces, where the zinc coating is disrupted by the welding process.
More particularly, it is an object of this invention to provide a resistance welding method for a zinc-coated steel component that forms in-situ a transient, nonferrous metallic liquid during welding that flows into fissures within the fused steel and, upon solidification, hermetically seals the weld. Furthermore, the method forms a corrosion inhibitive coating protecting the steel at external surfaces where the zinc coating is disturbed by the welding process. The method of this invention does not require an increase in welding electrical energy and may possibly extend electrode life.